1. Field of the Invention
The present invention relates to an optical wavefront sensor and more particularly to an optical wavefront sensor which incorporates optical and electronic heterodyning to enable high accuracy and high speed phase measurements to be made, relative to known optical wavefront sensors.
2. Description of the Prior Art
Wavefront sensors are known to be used to correct for distortions in optical beams caused by, for example, atmospheric aberrations. In particular, such wavefront sensors are known to be used with high power laser weapon systems, for example, as disclosed in commonly owned U.S. Pat. No. 5,198,607. The effectiveness of such laser weapon systems depends on many factors including the power of the laser at the target. Atmospheric aberrations are known to cause distortion of the wavefront of high powered laser beams and thus reduce the power and effectiveness of such weapons. As such, systems are known which predistort the wavefront to compensate for atmospheric aberrations so that maximum laser power is delivered at the targets.
Examples of wavefront sensors are disclosed in commonly owned U.S. Pat. Nos. 6,229,616 and 6,366,356. These wavefront sensors are based upon optical heterodyning a reference optical signal with an optical test signal. More particularly, an electro-acoustical device, such as a Bragg cell, is used to frequency upshift an optical reference signal. The optical test signal and frequency upshifted optical reference signal are then optically combined, which results in optical heterodyning of the two optical signals. The resulting optically heterodyned signal has a frequency equivalent to the beat frequency of the two signals, the RF signal driving the Bragg cell, normally in tens of MHz. The optically heterodyned signal is subsequently directed to a detector which converts the optical signal to an electronic signal having the same phase as the optical test signal. The electronic heterodyned signal is then used to develop a compensation signal to compensate for phase distortion in the original optical test signal. More particularly, the output of the photodetector is a sinusoidal output with a phase equivalent to the original optical phase. A heterodyne signal processor is used to convert the sinusoidal waveform into a plurality of pulse trains whose duty cycles are proportional to the sampled optical phase. These pulse trains are electronically integrated by a low pass filter in order to develop a DC voltage that is proportional to the duty cycle and to the phase of the optical test signal.
There are several problems with such known optical heterodyne wavefront sensors. First, such wavefront sensors are relatively slow due to the need to integrate the pulse trains from the optical heterodyne processors. In addition, known acoustical optical devices, such as Bragg cells, normally frequency shift at frequencies in the tens of MHz. However at these frequencies, the electronic jitter of approximately 1 nanosecond of the devices used for edge detection can be a source of substantial phase measurement noise. Thus, there is a need for a optical wavefront sensor which is faster than known optical wavefront sensors while virtually eliminating electronic jitter.